Battle of Huaibei

The Battle of Huaibei occurred in 294 AD during the War of the Eight Princes when Prince Sima Yue of Donghai attacked Huaibei, the capital of Prince Sima Wei of Chu. Sima Wei declared war on Sima Yue due to his concern about Sima Yue's expansion against neighboring minor princes, but he missed his chance to strike at Sima Yue's undefended southern towns and instead found himself fighting to defend his own home territory. Sima Wei had the larger army, but Sima Yue's forces outflanked his and slew his generals, leading to a rout of Sima Wei's army and the fall of his capital. Sima Wei, who was wounded in the battle, was forced to flee to his main army near Luoyang, depriving him of his territory and ending his war with Sima Yue.

Background
In 293 AD, the powerful Jin prince Sima Wei, Prince of Chu - fearful of his cousin Prince Sima Yue of Donghai's conquests - declared war on him and attempted to attack his unprotected southern lands as he campaigned against Sima Yao in the north. However, Sima Yue easily crushed Sima Yao in the Battle of Mengyin before going on to turn south with his army and march on Sima Yue's base in Chen Commandery. Sima Yue's 1,400-strong army arrived in Chen Commandery in the spring of 294 AD, besieging Sima Wei's capital at the farming town of Huaibei. Sima Yue was at a numerical disadvantage, and his advisors warned him that he would suffer a valiant defeat should he engage Sima Wei in battle. However, Sima Yue resolved to destroy the southern threat to his rule and bring an end to his ongoing wars.

Battle
Sima Yue, his wife Princess Pei, and his general He Lun positioned their army on a ridge across a vast field from another ridge where Sima Wei and his generals Sima Pu and Bo Shanan positioned their 2,500-strong army. Sima Wei's army prepared for a defensive battle, so Sima Yue marched his army out to the plain below Sima Wei's positions. He lured the enemy into a trap by sending out his cavalry to advance from his left flank and feign a wheeling maneuver towards Sima Wei's flank, leading to Sima Wei repositioning his army. As Sima Wei's men broke formation, Sima Yue ordered a general assault, and his infantry units attacked Sima Wei's army on multiple fronts. The cavalry then charged around Sima Wei's flank and attacked several of his archers and infantrymen from behind, leading to the collapse of Sima Wei's right flank. The battle was soon fought on two fronts; the larger battle was fought on the ridge where Sima Yue, his generals, and his cavalry attacked Sima Wei's main force, while Sima Wei's advance units met with the majority of Sima Yue's infantry in the middle of the field. As Sima Wei's main units began to rout, Sima Yue redirected some of his cavalrymen to attack Sima Wei's advance units from behind. Sima Wei was wounded in the battle, while both of his generals were slain, demoralizing his army. Before too long, Sima Wei's army collapsed into a rout, and Sima Yue's army pursued them and took several captives. The victory at Huaibei conquered Sima Wei's lone territory and destroyed his defensive army, while Sima Wei later recovered and returned to his main army near Luoyang, slightly beyond Sima Yue's reach.

Aftermath
Sima Yue's victory over Sima Wei ended all of the wars with his neighbors, and he was careful to ensure that any further expansion would not be met with flank attacks. After Sima Jiong marched an army through Sima Yue's lands in order to return to his home base in Shandong, Sima Yue cautiously formed a coalition with Sima Jiong, nominally to fight against barbarians. Having secured his northern flank, Sima Yue was now able to focus on expanding westward against Cao Huan or southward against Sima Jian.